The art has produced a number of integrated isomerization-adsorption systems for isomerizing a feed stream containing normal and non-normal hydrocarbons and producing a product stream which is useful as a gasoline blending feedstock.
Most of the prior art systems totally isomerize the normals in the feed and are referred to in the art as total isomerization processes, i.e., TIP. Among these are reactor-lead systems, where the fresh feed and any recycle is fed to the isomerization reactor prior to separation of non-normals, and adsorber-lead systems, where the fresh feed is fed to the adsorbers prior to isomerization. As currently operated, both of these schemes typically employ three or four adsorber beds which are cycled through at least one adsorption stage and two desorption stages.
In Canadian Patent No. 1,064,056, Reber et al describe a total isomerization process wherein large fluctuations in the concentration of either n-pentane or n-hexane in the reactor feed are prevented by suitably controlling the operation of a three-bed adsorber system. According to the disclosure, no more than two beds are being desorbed at any given time and the terminal stage of desorption in one of the three beds is contemporaneous with the initial stage of desorption in another of the three beds.
Both adsorber-lead and reactor-lead processes are specifically exemplified. The adsorber-lead process calls for first separating hydrogen from the reactor effluent. Fresh feed is then combined with the reactor hydrocarbon effluent and the combined stream is passed through the adsorbers to remove non-normal hydrocarbons so that the feed to the reactor is essentially normal hydrocarbons. This requires heat exchange equipment of significant size to handle streams of widely varying molecular weight and large energy inputs to cool the entire reactor effluent to separate the hydrocarbon and hydrogen portion, and then to reheat both. The reactor-lead process is similar in this regard.
In U.S. Pat. No. 4,210,771, Holcombe describes a reactor-lead total isomerization process which reduces the recycle rate to the reactor while maintaining a sufficient reactor hydrogen partial pressure by reducing fluctuations in hydrocarbon flow rates to the reactor. However, this reactor-lead process required cooling the entire reactor effluent to separate hydrogen from hydrocarbon portions prior to separating the normals from non-normals in a four-stage adsorption section.
The effluent from the isomerization reactor is condensed to separate a hydrocarbon fraction. This fraction is then reheated and passed as feed in the vapor state and at superatmospheric pressure periodically in sequence through each of at least four fixed beds of a system containing a zeolitic molecular sieve adsorbent having effective pore diameters of substantially 5 Angstroms, each of said beds cyclically undergoing the stages of:
A-1 adsorption-fill, wherein the vapor in the bed void space consists principally of a non-sorbable purge gas and the incoming feedstock forces the said non-sorbable purge gas from the bed void space out of the bed without substantial intermixing thereof with non-adsorbed feedstock fraction;
A-2 adsorption, wherein the feedstock is cocurrently passed through said bed and the normal constituents of the feedstock are selectively adsorbed into the internal cavities of the crystalline zeolitic adsorbent and the nonadsorbed constituents of the feedstock are removed from the bed as an effluent having a greatly reduced content of normal feedstock constituents;
D-1 void space purging, wherein the bed, which is loaded with normals adsorbate to the extent that the stoichiometric point of the mass transfer zone thereof has passed between 85 and 97 percent of the length of the bed and the bed void space contains a mixture of normals and non-normals in essentially feedstock proportions, is purged countercurrently, with respect to the direction of A-2 adsorption, by passing through the bed a stream of a non-sorbable purge gas in sufficient quantity to remove said void space feedstock vapors but not more than that which produces about 50 mole percent, preferably not more than 40 mole percent, of adsorbed feedstock normals in the bed effluent; and
D-2 purge desorption, wherein the selectively adsorbed feedstock normals are desorbed as part of the desorption effluent by passing a non-sorbable purge gas countercurrently with respect to A-2 adsorption through the bed until the major proportion of adsorbed normals has been desorbed and the bed void space vapors consist principally of non-sorbable purge gas.
This process results in wide fluctuations in the molecular weight of the adsorption effluent, has considerable complexity and requires all recycled hydrocarbons and hydrogen to be cooled and reheated.
There is a present need for improvements in isomerization-adsorption systems which will reduce energy consumption while preferably reducing adsorber bed volume and the overall complexity of the adsorption section.